1. Field Of The Invention
This invention pertains to the collection of seismic data and more specifically to high channel count collection and transmission of such data in an efficient manner.
2. Description Of The Prior Art
The importance of vertical seismic profiling and the obtaining of cross borehole data makes it imperative that the data collection and transmission technique be as efficient as possible to provide the maximum amount of data in the minimum amount of collection time. One of the main reasons for this is that the borehole being used for the collection of such data is taken out of service or production, similar to the situation when providing well-logging service to the well.
The economics of vertical seismic profiling ("VSP") and cross borehole recording would be more attractive if the amount of time needed to record data could be reduced. Companies who record VSP data charge several tens of thousands of dollars for their service, and these service charges are roughly proportional to the amount of time needed to record the data. More importantly, if data are recorded while a drill rig is still on site, then drill rig standby charges must also be paid.
In conventional vertical seismic profiling or cross borehole recording, a seismic source is located external to the borehole where the profiling data is to be gathered, usually being positioned on the surface or in another borehole. A receiver is located in the borehole to receive the seismic data signals. The received pulses are each converted by a suitable transducer to an analog signal and either transmitted in that form or in digital form to the surface for storage. The receiver is then elevated to another vertical location and the procedure is repeated.
One effective way to reduce VSP field recording time is to record data at more than one depth for each seismic shot. Recording VSP data at even two or three depth levels simultaneously would result in an appreciable reduction of recording time over that now required to collect data with current single-point recording systems. However, it is not readily apparent from such observation how to implement the collection of data, its handling and treatment, and the transmission thereof in an efficient and simple manner within the physical constraints found in a downhole environment.
Multi-level VSP receiver systems have been used in the Soviet Union for several years. A discussion of these Gal'perin, a translated publication of the Society of Exploration Geophysicists, copyright 1974, as Special Publication No. 12. The primary techniques used to transmit data in these systems described in this publication have been amplitude and pulse-width modulation. Each receiver's signal modulates the amplitude or width of a high frequency (approximately one kilohertz) carrier that is on the data lines at all times. The carrier frequency is different for each receiver. At the surface, the carriers are isolated and the signals recovered by conventional methods. The electronics required for this technique are far from trivial. The limiting factor is crosstalk in the wireline; that is, the number of different carrier frequencies which the cable can sustain without mixing the individual signals. Devices with up to twelve channels have been reported in the literature.
A similar method for recording multi-level VSP data was recently developed at Sandia National Laboratories. The seismic signals from eight downhole receivers are frequency division multiplexed onto frequency modulation subcarriers for transmission to the surface through a single conductor wireline. Again, the electronics of this system is not simple and the number of channels is limited by crosstalk.
The most popular acquisition method currently used for multi-level VSP is downhole digitizing. Downhole digitizing places separate analog-to-digital converters and memory on each of several receiver sondes. A sonde may handle data from one or from a few receivers. After a shot, the sondes are polled and transmit their digitized data uphole sequentially, using only two conductors. This technique uses relatively expensive and complicated electronics which can be quite sensitive to the borehole temperature. CGG American Services, Inc. offers this service (Multilock) with a maximum of four sondes comprising twelve channels of vertical receivers. Schlumberger Well Services is also marketing a system (Downhole Seismic Array) with eight channels.
There are also superficial similarities in VSP and well-logging that warrants a review of the possible applicability of well-logging apparatus as VSP apparatus or in VSP procedures. There are some instances in the well-logging prior art, for example, of the use of multiple acoustical receivers, where the typical acoustical well-logging frequencies are on the order of 5-25 KHz. Such frequencies are much higher than the 20-200 Hz frequencies used in seismic profiling. Nevertheless, as is shown for example in U.S. Pat. No. 4,684,947, Zimmer, a tool is disclosed having an acoustic transmitter and four receivers, one above the other, in a common sonde with a transmitter. The return pulses are received, converted to analog signals, multiplexed, converted to digital form and then transmitted to the surface for recording and later processing. There are many other examples in the well-logging literature using multiple receivers and utilizing similar electronics for polling the receivers and digitizing the data for uphole transmission using the two conductors in a typical sevenconductor cable that are normally available for data transmission purposes.
It is well-known that the industry standard logging cable includes seven conductors. Since two of the conductors are used for power, one is used for ground, and two are used for control lines, all data must be transmitted on the two remaining conductors. It is possible when there is only one receiver to transmit the received signals in their analog condition are as described above; however, when multiple receivers are used, digital conversion has been commonly employed, such as in the embodiment shown in the '947 patent, because of the constraints of the cable transmission bandwidth.
Although the multiplexing and digitizing electronics that have been employed with the higher frequencies used with acoustic well-logging are satisfactory in such application, such electronics are subjected to high temperatures and often result in data warp that is unacceptable at the frequencies employed in seismic profiling, i.e., in the range between 20-200 Hz.
Returning to prior art VSP apparatus, it should be noted that in land based exploration the individual VSP geophones that are employed are housed in sondes, which are positioned in the well borehole by suspending them from the logging cable at the desired depth and then clamping them to the borehole wall. Most current sondes are designed to clamp to the borehole wall by means of a single motor or hydraulically driven arm mechanism that extends from one side of the body of the sonde to one side of the well bore, thereby forcing the body of the sonde against the opposite side of the well bore. The VSP geophone is effectively coupled to the well bore by a single force in this manner. Another common technique that is employed is to clamp the sonde to the borehole wall with powerful electromagnets. This technique is limited in application, however, to wells with boreholes lined with metal casing.
Clamping the geophone to the borehole wall allows the logging cable, by which the geophone is suspended, to be slackened so that it no longer supports the weight of the geophone and thereby instantly removes one major transmission path for extraneous noise known as cable waves. The degree of clamping to the borehole wall is an important factor if the geophone response is to be without distortion and proportional to the small amounts of particle displacement of the rock surrounding the borehole. There must be no differential movement, i.e., slippage, between geophone and wall during passage of the seismic event; this means in practice that for a mechanically coupled sonde a lateral force equivalent to about two or three times the weight of the geophone must be supplied by the arm mechanism. Typical VSP tools weigh about 150 pounds. This implies a locking mechanism capable of delivering about 200 to 300 pounds of lateral force if reasonable coupling is to be achieved.
Therefore, it is a feature of the present invention to provide improved vertical data collection apparatus that allows high channel count collection and fast data transmission using two-wire cable conduction.
It is another feature of the present invention to provide improved data collection and transmission wherein the transmission of the detected analog signals is compartmentalized in a time multiplexing sequence, thereby allowing any required digitizing to be performed at the surface and avoiding performing digitizing operations downhole in the geophones or below the surface in hydrophones.
It is another feature of the present invention to provide distributed time multiplexing of received seismic signals at a plurality of receivers vertically aligned in a borehole, wherein the overall pulse period of the transmitted seismic pulse is on the order of about 20 milliseconds, and wherein the total multiplexing period is only on the order of about 15 microseconds, thereby providing sampling of the received seismic pulse on the order of over 1000 times each pulse period to thereby make the data information of an overall seismic pulse received at each receiver of high fidelity to the actual received pulse.
It is still another feature of the invention to provide distributed time multiplexing of received seismic signals compatible for use with downhole digitizers while not requiring such digitizers.
It is also a feature of this invention that the downhole technique of time-multiplexing of analog signals can be adapted for use with hydrophones in an aquatic environment or for use with geophones distributed on the surface.
It is yet another feature of the present invention to provide improved coupling of an electronic receiving apparatus to the well bore so as to provide suitable geophone reception of seismic signals, such as are used in VSP.